The present invention relates to an antenna for radio apparatus and more particularly to a compact antenna used for a compact portable terminal having a small occupation volume such as a portable mobile terminal.
An inverted F type antenna or a helical antenna has hitherto been employed for the compact portable terminals having a small occupation volume such as portable mobile terminals In either type of antenna, when the antenna volume is decreased as the miniaturization of the terminal advances, a capacitance component larger than a radiation resistance component takes place. In order to cancel this large capacitance component, the conventional antenna needs a matching circuit provided separately from an antenna proper. An example of the conventional antenna is disclosed in, for example, JP-B-2-22563.
In the conventional antenna, characteristics of the antenna having the antenna proper and the matching circuit in combination have to be studied and from a standpoint of occupation volume, the matching circuit forms a factor which limits the miniaturization. Further, the matching circuit is realized with lumped constant elements (inductors and capacitances) or transmission lines and upon incorporation of the antenna into the terminal, these elements must also be incorporated thereinto, thus considerably raising cost.